Insert an Null value into the database

I want to insert a null value into the database, but i keep getting annoying errors. Let me explain the situation first:

I am filling an ddl with values form the database. The column contains NULL values. The ddl isn't filled with these NULL values so I add them manually (ddlSubConsolidations.Items.Insert(0, "");). Now I want to add all the values to the database by the Button Click event:

    protected void btnAdd_Click(object sender, EventArgs e)
    {
// Some Validation rules
        if (ValidateName() && ValidateExists())
        {
// If the selected value in the ddl is empty the column needs to be filled with an Null value.
            if (ddlParent.SelectedValue.ToString() == "")
            {
                FireInsert(DBNull.Value.ToString());
            }
// Else the selected value has to be inserted into the database.
            else
            {
                FireInsert(ddlParent.SelectedValue.ToString());
            }
        }
    }

    private void FireInsert(string CheckedDdlParent)
    {
        dc.ExecuteSQL(queries.InsertConsolidation(CheckedDdlParent, txtName.Text, ddlCurrency.SelectedValue.ToString()));
    }

Now I get the error. Does somebody have any ideas how to get a Null value in the database by using this code?

I know that I can execute the insert in the if and the else, but I want to know if its possible this way.

And sorry for the bad written english :-$ .